The development of a vaccine regimen capable of eliciting broad neutralization of HIV vaccine strains remains an important but elusive goal. HIV-specific antibodies can be raised through the use of live vector vaccines, subunit protein vaccines, and other approaches, but the generation of significant breadth of neutralization against primary isolates of HIV has not yet been demonstrated in a human vaccine trial. A recent trial performed by the HIV Vaccine Trials Network (HVTN 049), in which volunteers received a DNA prime and a recombinant, trimeric, V2-deleted, clade B envelope protein product from Novartis as a boost, is a good example of a trial that generated high levels of neutralization against the homologous and easily-neutralized SF162 strain but yielded very little breadth against Tier 2 isolates. It will be helpful in designing future trials and immunogens to understand how the B cell response in vaccinees compares to that seen in the subpopulation of HIV-infected individuals who do develop significant neutralization breadth. The major goal of the present application is to apply state-of-the-art B cell repertoire and monoclonal antibody technologies to the analysis of responses of human volunteers enrolled in HVTN 088, in which HVTN 049 participants are receiving booster immunizations with a clade C trimeric protein. This study will define the B cell subsets involved in the vaccine response, the degree of affinity maturation present in antibody variable gene segments, the length of the CDR3 loops generated, and the neutralization epitopes targeted in vaccine recipients who have been primed with the clade B protein years before in comparison with those who are unprimed. Monoclonal antibodies will be generated from selected recipients to further define the nature of breadth present and to test the hypothesis that cross-neutralizing monoclonal responses can be elicited through vaccination. These data will provide important new information that will inform the design of HIV vaccine regimens designed to elicit broadly- neutralizing antibodies.